This invention relates to a solid-state image pickup device.
Solid-state image pickup devices take out signal carriers produced at the junction portion provided at every pixel on the semiconductor substrate surface to transfer them, and therefore provide a pictorial image signal.
In such solid-state image pickup devices, it is important in improving the picture quality, to effectively prevent dark currents by the signal carriers which are produced even if no light is incident thereto.
An approach is disclosed in Japanese Patent No. 1325458 by the assignee of this application, to form an inverting layer at the surface portion of the photosensitive pixel where the dark current is produced to the maximum degree, thereby permitting the dark current to be drastically reduced. For this purpose, a storage electrode is provided on the photosensitive pixel through an insulating film. A bias power source is connected to the storage electrode. When a negative voltage is applied to the storage electrode by the bias power source, an inversion layer is formed at the surface portion of the photosensitive pixel. Thus, the occurrence of the dark current is prevented.
Since it is required that incident light pass through the storage electrode to reach the photosensitive pixel, the storage electrode must be transparent. However, the electrode materials which can be formed most easily and stably at present are not necessarily transparent. For example, where a polycrystalline silicon electrode is used, light with a short wavelength (blue light) is remarkably absorbed, with the result that the so-called blue sensitivity is greatly reduced. For this reason, when an attempt is made to provide a color image pickup signal using a solid-state pickup device, the entire color balance must be adjusted by suppressing the medium- and long-wavelength sensitivities in harmony with the short wavelength sensitivity, resulting in the problem that the sensitivity of an apparatus using such a solid-state image pickup device is remarkably degraded.